1. Field of the Invention
The present invention relates to a method for estimating a road surface condition under a traveling vehicle.
2. Description of the Related Art
To enhance the travel stability of a vehicle, it is desired that the road surface condition under a traveling vehicle is estimated with accuracy and the data thus obtained is fed back to vehicle control. If the road surface condition can be estimated in time, then it will be possible to operate such advanced control as ABS (antilock braking system) braking before taking any danger avoidance action such as braking, accelerating, or steering. With such facility, there will be a marked boost in the safety of vehicular operation.
In a proposed method for estimating a road surface condition (see Patent Document 1, for instance), a time-series waveform of vibration of the tire tread of a traveling vehicle is detected by an acceleration sensor installed on the inner liner of the tire. The time-series waveform is then subjected to a frequency analysis by extracting the time-series waveform of a domain including a leading end point of tire contact patch (footprint) and the time-series waveform of a domain including a trailing end point. From frequency spectrums thus derived, a band value Pfi of the leading end domain and a band value Pkj of the trailing end domain, which are the vibration levels of the plurality of frequency bands, are calculated respectively. And a road surface condition is estimated from these band values Pfi and Pkj.
It should be noted that the positions of leading end point and trailing end point of tire footprint can be identified from the positions of peaks appearing in the time-series waveform of tire vibration. However, when the tire receives an excessive input (impact), such as when going over a bump or a curb, a conspicuous peak appears in the time-series waveform. In such cases, the positions of leading end point and trailing end point can sometimes be estimated incorrectly.
A solution to this problem as disclosed in Patent Document 1 is the installation of an acceleration sensor for monitoring on the suspension. And when the value of acceleration detected by the acceleration sensor for monitoring exceeds a predetermined threshold value, it is determined that there has been an excessive input (hereinafter referred to as large input) to the tire, and the estimation of a road surface condition is canceled.